Recently, there has been a growing demand for solid polymer electrolyte fuel cells. In producing e.g., membranes and electrodes for solid polymer electrolyte fuel cells, a solution of fluorine-based polymer electrolytes (hereinafter referred to also as “fluorine-based polymer electrolyte”) having a sulfonate functional group (hereinafter referred to also as “H-type”) is used.
It is required that these electrolyte membranes and electrodes have hot water dissolution resistance in order to improve output characteristics of fuel cells. It is also required that a solution having an electrolyte highly dispersed therein is produced in a short time. In addition, as a material for an electrolyte membrane and an electrode a highly concentrated electrolyte solution is preferable in view of handling.
In conventional methods for producing a fluorine-based polymer electrolyte solution, a fluorine-based polymer electrolyte is generally dissolved in a water/alcohol mixed solvent while stirring in a batch-type closed reactor, such as an autoclave electrolyte under high temperature and pressure.
For example, Patent Literature 1 discloses a method of dissolving a bulk of a perfluorosulfonated polymer electrolyte in a water/ethanol mixed solvent at 165° C. for 7 hours to solid content of approximately 5 mass % solution, in an autoclave made of SUS304 having a glass inner cylinder.
Patent Literature 2 discloses a method of supplying a micron-order fine-particles dispersion without clogging by controlling the angle of a flow channel.
Patent Literature 3 discloses a method of treating an electrolyte-containing solution with heat at a temperature of the glass transition temperature of the electrolyte to 300° C.
Patent Literature 4 discloses a method of suspending organic and inorganic components in water, bringing the water into a near-critical or supercritical state and passing the aqueous solution of this state through a tubular reactor.
Typical examples of the H-type fluorine-based polymer electrolyte solution include Nafion <registered trade mark> Dispersion Solution (manufactured by DuPont in the United States) and Aciplex <registered trade mark> —SS (manufactured by Asahi Kasei E-materials Corporation). However, since the solubility of the H-type fluorine-based polymer electrolyte to a solvent is extremely low, various techniques have been so far proposed for use in a method for producing an electrolyte solution.
For example, Patent Literature 5 discloses methods of dissolving bulks of H-type and sodium-type (hereinafter referred to also as “Na-type”) fluorine-based polymer electrolytes in a solvent containing water or a water-immiscible organic solvent at a high temperature of 200° C. or more.
Patent Literature 6 discloses a method of dissolving a bulk of a Na-type fluorine-based polymer electrolyte in water at a high temperature of 200° C. or more. Patent Literature 7 describes a method of dissolving an emulsion of a fluorine-based polymer electrolyte by heating while stirring in an autoclave at a temperature of 50 to 250° C. for 1 to 12 hours.